In conventional head space sample analysis, a liquid or solid sample is contained in a vial that is connected to a head space sampling device. The head space sampling device is used to sample the head space above the sample within the vial. Often, the sample is heated to produce a vapor that fills the head space. Prior to sampling of the head space, the vial is often pressurized with a gas that is provided to the vial in a controlled manner. For example, the vial typically can be pressurized by a gas until a selected pressure is reached. When desired pressure characteristics are present within the head space sampling device, the gas from the head space can be directed toward a head space analyzer.
Conventional head space sampling devices have a sample loop for receiving portions of a gas sample within the head space of a vial. These head space devices typically have a ventilation valve for venting gas from head space through the sample loop to the external pressure environment, thereby drawing portions of the gas sample from the vial to the sample loop. However, existing head space sampling devices cannot be programmed to precisely control the venting of the gas from the head space through the sample loop to the external pressure environment. Consequently, the amount of the gas sample that is transferred from the vial to the sample loop can vary significantly from sample to sample. Furthermore, sudden pressure changes resulting from venting of the gas from the head space through the sample loop to external pressure can shift any liquid/vapor equilibrium that was previously established within the system, thereby leading to inconsistent results. Additionally, venting of the gas from the head space to an undesirably low pressure can lead to insufficient pressures within the sample loop, and, therefore, insufficient gas sample concentrations within the sample loop.
Some conventional head space sampling devices have back pressure regulators for actively maintaining the pressure within the vial and the sample loop at a desired level. However, these devices require an additional gas source to actively regulate the vent pressure. Additionally, after the vial is pressurized, the opening of the ventilation valve can cause the pressure within the vial and the sample loop to rapidly decrease to the pressure set by the back pressure regulator. This rapid decrease in pressure makes it difficult to deliver consistent amounts of gas to the sample loop and the head space analyzer, leading to poor area repeatability during head space analysis.
Accordingly, there is a need in the pertinent art for automated and programmable systems and methods for depressurizing gas within a head space in a gradual, controlled manner to thereby maintain consistency in the gas sample concentrations that are sent to the gas analyzer. There is a further need in the pertinent art for automated and programmable systems and methods of depressurizing gas within a head space at a desired rate and to a desired pressure appropriate for any sample loop volume.